[llvm] [WebAssembly] Misc. fixes in CFGStackify (PR #107182)

[Heejin Ahn via llvm-commits]

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@@ -359,21 +360,20 @@ void WebAssemblyCFGStackify::placeBlockMarker(MachineBasicBlock &MBB) {
               TII.get(WebAssembly::BLOCK))
           .addImm(int64_t(ReturnType));
 
-  // Decide where in Header to put the END_BLOCK.
+  // Decide where in MBB to put the END_BLOCK.
   BeforeSet.clear();
   AfterSet.clear();
   for (auto &MI : MBB) {
 #ifndef NDEBUG
-    // END_BLOCK should precede existing LOOP and TRY markers.
-    if (MI.getOpcode() == WebAssembly::LOOP ||
-        MI.getOpcode() == WebAssembly::TRY)
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aheejin wrote:

We place `loop` markers first for the whole function, and then place `try`/`block` in one pass while we traverse BBs from top to bottom:
(Long ago we used to place `try` first and then `block`, but this turned out to be not working, so I changed it to a single pass: https://github.com/llvm/llvm-project/commit/44a5a4b10704b336700a94403103faaba6fa9729)
https://github.com/llvm/llvm-project/blob/6c607cfb2c2d8acd2b92d7ed8106ab1e4fc0d79d/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp#L1542-L1557

So there can't be a `try` marker in a BB that we are about to place an `end_block` at this point. The reason we had this code is, we used to place `try` for the whole function first, which cannot happen anymore.

https://github.com/llvm/llvm-project/pull/107182